Reducing the excess voltage in electrolysis of aqueous hydrochloric acid in diaphragm cells



Aug. 15, 1967 R. HIRSCHBERG 3,336,209

REDUCING THE EXCESS VOLTAGE IN ELECTROLYSIS OF AQUEOUS HYDROCHLORIC ACID IN DIAPHRAGM CELLS Filed Oct. 6, 1964 LL (A F!) INVENTQR RMMHF HM$CWBERG BY MW ATTORNEY United States Patent 3 Claims. Cl. 204-128) The present invention relates to the electrolysis of aqueous hydrochloric acid in diaphragm cells.

When electrolysing aqueous hydrochloric-acid in diaphragm cells to obtain chlorine and hydrogen it is known to reduce the cell voltage by adding metal salts to the electrolyte. In the course of the electrolysis, such metals deposit on the cathodes of theelectrolytic cells. The layer thus produced reduces the hydrogen overvoltage. When using conventional conditions of industrial electrolysis, the hydrogen overvoltage is about 0.35 volt. Thus, the cell voltage is reduced for example from 2.35 volts to 2.00 volts, whereby electric energy is conserved. In order to reduce the hydrogen overvoltage in the electrolysis of hydrochloric acid, platinum, copper, nickel, antimony, silver, molybdenum and cobalt have been used. In order to obtain a satisfactory effect, large amounts of the aforesaid metals must be employed. For example, about 200 grams of SbCl or 1000 grams of Nichol-I 0, are required in a cell having a total cathode surface of m. Further, said additives have the drawback that the cathodic layers substantially reduce the hydrogen overvoltage for a short time only. After the addition of the metal salt, the cell voltage increases again during electrolysis so that finally the cell voltage prevailing before the addition of the salt is practically restored. Moreover, when platinum salts are used, layers of finely divided platinum are formed which are practically insoluble in the hydrochloric acid used for electrolysis purposes. These layers cannot be dissolved by the small amounts of chlorine formed on the cathode. In order to overcome this disadvantage, portions of the platinum layer which have dropped off the electrode may be withdrawn from time to time from the bottom of the electrolytic cell, dissolved in suitable manner and added again to the electrolyte. However, such a mode of operation presents considerable difiiculties owing to the usual filter press-like construction of the electrolytic cells containing narrow individual cells and therefore requires repeated and undesired interruptions of the electrolysis. Moreover, over an average time period only a relatively small saving in voltage is obtained by these known measures, since the hydrogen overvoltage can be signficantly reduced only for a short period of time because of the continuous degeneration of the layer. Subsequently electrolysis must be carried out with a voltage considerably greater than the minimum value theoretically obtainable (total voltage less overvoltage) until the layer is regenerated. Still further, frequent regeneration of the active layer on the cathode is impossible. When antimony is sometimes used, for example, a voltage-reducing effect cannot be reproduced after a repeated addition of SbCl to the electrolyte.

It has now been found that the aforesaid disadvantages are avoided by the addition of palladium salts, especially palladium chloride, to the electrolyte. With the use of relatively small amounts of palladium, cathodic deposits of long-lasting activity are formed. Thus considerable industrial and economical advantage is achieved.

The process according to the invention can be carried out by adding a soluble palladium salt, for example palladium chloride, to the electrolyte in an electrolytic cell. Advantageously the palladium salt is added in the form of a solution, and more advantageously in the form of a hydrochloric acid solution and in an amount of 0.2 to 2 grams, preferably 0.8 to 1.5 grams of palladium for 1 square meter of active electrode surface. After the addition of the palladium salt, the voltage in the individual cells drops practically by the amount of the hydrogen overvoltage, for example from about 2.35 volts to 2.00 volts with a load of 8000 amperes. After having been in continuous operation for several days, the voltage in the individual cells increases again to 2.15 volts. A further increase in voltage cannot be observed even after a prolonged operating time.

According to a particularly advantageous variant of the process of the invention, the palladium salt is added not all at once but continuously or periodically in small amounts. It has surprisingly been found that additions of about 0.5 to 1.0 milligrams of palladium per hour and square meter of cathode surface are sufficient to suppress practically completely the hydrogen overvoltage for any period of time. Consequently, a continuous electrolysis according to the process of the invention takes place without hydrogen overvoltage and a regeneration of the active layer is not necessary.

Since only very small amounts of palladium must be supplied continuously in this advantageous variant of the present process, the cost of the metal used corresponds with only a small fraction of the electric energy saved and, therefore, a recovery of the palladium used can be dispensed with. The amount of palladium deposited on the cathode surface after one year of operation is 5 to 10 grams of palladium per square meter so that the metal can be supplied over a very long period of time without the formation on the electrodes of thick deposits which would detrimentally affect the electrolysis.

It is possible, however, to recover the palladium added to the electrolyte in simple manner. This can be done at any time, for example during a routine overhaul. The palladium layer on the electrodes can be regenerated by a short disconnection of the cell. The palladium layer is then readily dissolved on the electrodes by the chlo- Iine-containing hydrochloric acid formed by diffusion of chlorine from the chlorine-containing hydrochloric acid of the anode space through the diaphragm into the acid of the cathode space. This process may be promoted by allowing further chlorine-containing hydrochloric acid to travel from the anode space through the diaphragm into the cathode space. When the current is then switched on again, the metal separates again as active layer on the cathode. In the case of large palladium deposits, the solution containing palladium salt can be collected in a separate vessel and subsequently added in portions to the electrolyte. Still further, it is possible to separate the palladium from the solution in the form of metal by a conventional method.

The process according to the invention offers the particular advantage that parts of the layer dropping from the cathode during operation dissolve in the acid of the cathode space. The latter always contains a small amount of chlorine because of the diffusion through the diaphragm. It is thus not necessary to work up the inactive cathode sludge. Moreover, with continuous operation the activity of the palladium layer produced is appreciably greater than that of the metals hitherto used for this purpose.

The following example serves to illustrate the invention but it is not intended to limit thereto. In the example the palladium is added in continuous manner.

3 Example The electrolytic cell used was-composed of 14 seriesconnected individual cells with bipolar electrodes between which a plastic diaphragm was inserted to separate the developed gases. The cathode and anode surface of the individual cell was 2.5 square meters, and the total cathode surface 35 square meters. 200 liters per hour of hydrochloric acid of about 23% strength were introduced in each cell into the space between anode and diaphragm or cathode and diaphragm, respectively. The gases developed during electrolysis and the depleted hydrochloric acid of about 20% strength by weight were withdrawn .at the head of the cell and separated from the liquid via siphons. The acids from the anode and cathode spaces were readjusted with hydrogen chloride to a content of about 23% strength by weight and recycled into the anode and cathode spaces.

In the annexed drawing the average voltage of an individual cell (total terminal voltage of the cell divided by 14) is plotted in curve I as a function of the current flowing through the cell. When 30 milligrams of palladium were added per hour to the anolyte of the cell in the form of a hydrochloric acid solution of PdCl after a certain starting time a current-voltage relation according to curve II was observed. The results of measuring indicated relate to measurements after 2, 8 and 12 days operation and show that the voltage reduction of about 0.35 volt per individual cell is independent of the operating time.

I claim:

1. In a process for reducing the hydrogen overvoltage in the electrolysis of aqueous hydrochloric acid in diaphragm cells to form hydrogen and chlorine by the addition of metal salts to the electrolyte, the improvement wherein said metal salt is a salt of palladium added in an amount providing from 0.2 to 2 grams of palladium per square meter of active electrode surface.

2. A process as in claim 1 wherein said polladium salt is palladium chloride.

3. A process as in claim 1 wherein said palladium salt is added in an amount providing from 0.8 to 1.5 grams of palladium per square meter of active electrode surface.

References Cited UNITED STATES PATENTS 1/1954 Low et a1. 204l28 4/1964 Teske et al 204128 

1. IN A PROCESS FOR REDUCING THE HYDROGEN OVERVOLTAGE IN THE ELECTOLYSIS OF AQUEOUS HYDROCHLORIC ACID IN DIAPHRAGM CELLS TO FORM HYDROGEN AND CHLORINE BY THE ADDITION OF METAL SALTS TO THE ELECTROLYTE, THE IMPROVEMENT WHEREIN SAID METAL SALT IS A SALT OF PALLADIUM ADDED IN AN AMOUNT PROVIDING FROM 0.2 TO 2 GRAMS OF PALLADIUM PER SQUARE METER OF ACTIVE ELECTRODE SURFACE. 